The present invention relates to a sheet of roof-covering material, comprising a base layer provided with apertures equally spaced over its surface.
Such a sheet is known in which the base layer by immersion is impregnated with bitumen and, on both sides, is provided with a bituminous coating.
In applying the sheet, the same is unrolled onto and over the roof plane, which may have been constructed from various materials, such as for instance wood or concrete, and after which hot and molten bitumen is spread out onto the sheet and caused to pass through the apertures to come into touch with and to adhere the sheet to the roof plane. After applying the sheet onto the roof plane, a top layer of bituminous material is unrolled onto and adhered to the upper side of the sheet to complete the roof-covering.
The known sheet of roof-covering material as described here above has the disadvantage of being impermeable for liquid and vapor, so that moisture enclosed between the sheet and the roof plane only can escape underneath the sheet to the surroundings and for which reason a granular material, such as fine gravel, may be applied underneath the sheet to keep it locally in spaced relationship with respect to the roof plane.
However, such granular material rather easily may penetrate into the roof-covering so that there is no way out any more for moisture enclosed underneath the sheet and which moisture, when being warmed up by sunshine, will cause the formation of blisters in the roofcovering.
Another disadvantage of the sheet known from the prior art is, that the hot and molten bitumen spread out onto the sheet to adhere it to the roof plane has to be cooled down before the top layer may be unrolled onto and over the sheet to complete the roof-covering.
It is an object of the present invention to obviate these disadvantages of the known sheet of roof-covering material.